Medical acoustics covers a wide variety of applications that are both diagnostic and therapeutic. One of the interesting aspects of acoustics is its capability to be apparently noninvasive (imaging of the fetus with ultrasound) and yet powerful enough to generate biological effects (destruction of kidney stones by shock waves). One new area which seems to be further expanding this capacity is ultrasound contrast agents. Most of the agents under development are very small, stable gas bubbles which can be injected into the bloodstream and still pass through the capillary beds. Some agents provide additional signals in medical imaging for several minutes, and some have the potential for therapeutic applications as well. Contrast agents are one of the fastest growing areas in medical acoustics, as evidenced by the increased research in this area and the number of major pharmaceutical manufacturers becoming involved. Research and development expenditures are expected to increase dramatically as a result and the potential impact on medical acoustics could be significant. Topics for discussion include detection of nonlinear signals, new imaging modes in medical ultrasonics, the ``disappearance'' or ``reappearance'' of agents when exposed to an acoustic field, potential bioeffects of agents, and their therapeutic applications such as use in drug delivery.